Such fluid power actuators are presently used in very great numbers in many different sorts of machines as linear driving motors because they take up little space and are troublefree in operation. One shortcoming however is that, more specially in the case of air power actuators, the piston and piston rod are likely to be moved under the effect of outside forces upon the occurrence of an air supply failure. Such uncontrolled motion is more specially undesired if the loads moved by a number of different actuators are to be pushed along a common path and may get in each others way if, because of such a failure, the actuators are no longer timed in the way desired.